1. Field of the Invention
The present invention relates to devices for facilitating orthopedic procedures. More particularly, this invention pertains to a tool for use in centering the travel of an elongated surgical tool, such as a drill bit, to thereby avoid perforation of bone tissue.
2. Description of the Prior Art
A number of important orthopedic surgical procedures require the use of a potentially-harmful elongated cutting tool, such as a drill bit, at a distance removed from the surgeon's ready control. Such procedures include the revision of hip protheses, and fixation of acute fractures and non-unions that routinely require the clearing of a channel within a long bone. For example, FIG. 1 is a side elevation view, partly in section, of a hip replacement including a prosthesis 10 fixed within a human femur 12. The prosthesis is fabricated, for example, of stainless steel and includes a metal ball or prosthetic femoral head 14 coupled to a prosthetic hip socket (not shown). The lower portion of the prosthesis 10 extends to and terminates in an elongated stem 16. The stem 16 extends into the femoral canal where it is anchored to the bone by means of an appropriate cement 18 (a mixture of a monomer and a polymer).
The hardened cement 18 surrounds the stem 16 extending beneath it to form a solid region within the surrounding wall of bone tissue known as a cement "plug" 20. Optionally, a stop or restrictor 22 of plastic composition is fitted within the femur 12 for increasing the backpressure on the cement 18 as it hardens and improving the quality of the cement-to-bone tissue bond.
Hip surgeries of the type illustrated in FIG. 1 presently experience useful lives of from 10 and 20 years. Gradual breakdown or loosening of the bond between the cement and bone tissue occurs over time, causing the patient to experience pain. When such pain becomes significant, a redoing of the former replacement is indicated in which the existing prosthesis is removed and a new prosthesis attached in its place. The required surgical procedure involves removal of the hardened cement and optional restrictor in addition to the former prosthesis, prior to insertion of the new device. The second hip replacement procedure often employs a non-cement fixation technique due to the thinning and reduced adhesion of existing bone tissue. The latter prosthesis may, for example, be formed with beads or mesh, providing a surface that encourages interdigitation of bone tissue thereon. An alternative technique is to use cement again injected into and pressurized within a femoral canal cleared of all debris.
The hip prosthesis substitution is begun by removal of all foreign objects from the femoral canal. The actual prosthesis may be readily removed during surgery, leaving the mantle of hardened cement and possibly a plastic restrictor. Removal of the structures associated with the former prosthesis becomes more complicated and risky as one progresses further and further from the top or head of the bone into the femoral canal. The uppermost cement is not terribly difficult for the surgeon to remove due to good visibility and accessibility. In addition, the removal procedure is facilitated in this region by the existence of a large central channel or void left after removal of the stem of the prosthesis. Such evacuated area extends the region of access and visibility, and thus the work region, well into the femoral canal. A number of techniques are useful in this area, including the use of power burrs, hand-held chisels and ultrasound (to "melt" the cement). All are reasonably satisfactory for removing the cement mantel from surrounding bone tissue. As one proceeds further to the region of the cement plug, discussed above, visibility and accessibility limitations begin to crop up. Maintenance of adequate lighting is difficult and further obscuration is caused by the increased presence of blood.
The nature of the surgeon's work is complicated by the fact that the plug region of the cement mantle and the optional restrictor lie beneath the access channel or void left by removal of the prosthesis. A number of techniques are commonly employed to remove the cement plug and restrictor. Some involve the use of elongated drill bits, burrs, or hand-held chisels to break through, fragment or create a hole in the cement plug. An elongated tool having a hooked end can then be inserted through the obstruction and an upwardly directed force applied to lift and remove these remaining obstructions. Alternatively, if a drill hole has been made, a device may be threaded into the hole with a similarly directed force applied.
Visibility and alignment difficulties in the plug region create a very dangerous environment in which to operate a drill bit, burr or chisel. Misalignment of these elongated instruments may result in the perforation of bone tissue. The surgeon, however, must remove (or displace distally beyond the tip of the new prosthesis) all cement from the prior prosthesis (along with the plastic stop) despite the potential dangers. Leaving cement can adversely affect preparation of the femur. Other instrumentation as well as the prosthesis may be misdirected by any remaining cement mantle. In addition, the remaining surface may be incapable of providing good adhesion.
In order to overcome the serious perforation risk, x-ray or like equipment is often employed to assure centering of the potentially-harmful tool with respect to the surrounding bone tissue. Such equipment is bulky and therefore creates difficulties for the surgeon who must work within a limited area. Additionally, this equipment can act as a source of contamination. Another method involves excising a viewing "window" in the bone. This, of course, complicates the operation, increasing its duration, the loss of blood and the risk of eventual bone fracture.